Master Naturalists: Walking fish a missing link

By Paul and Mary Meredith
April 17, 2014 at midnightUpdated April 16, 2014 at 11:17 p.m.

Want to see what Tiktaalik roseae looked like? In doing research for this article, we discovered that PBS is going to offer a three-part TV series about "Your Inner Fish", moderated by Dr. Shubin in April 2014. Check your local listings for times or go to PBS.org for information.

We're in California visiting the new granddaughter. Walking with Gracie the dog, we spotted a dead bird. Ants had worked it over; it was mostly feathers and bones, including one wing. Looking at the wing, Paul started mulling over things he knew about wings. There was a shoulder, then one bone connected to two bones and then a couple of bones that sort of looked like fingers.

Hollow and light, birds' wings are flapped by powerful breast muscles. That got him thinking about an e-book he bought a couple of years ago about the search for a missing evolutionary link between fish and higher animals, by University of Chicago organismal biology and anatomy professor Neil Shubin.

Anatomists know all limbs, including wings, have one bone like the arm's humerus or the leg's femur, then two bones, like the forearm's radius and ulna, connected by a joint. Next are several bone "blobs" (Shubin's term) connecting to fingers or toes. How creatures' limbs vary on this basic pattern is in the upper three bones' length, the number of blobs, how the blobs are fused and the number, length, and strength of the fingers.

Us and bats each have five fingers, but the bat has four really long, thin ones with skin between them forming a wing. While we look not at all like a horse, we both follow the same plan.

Modern horses have only two toes per leg while fossil horses show vestiges of other toes that later disappeared because they were not necessary for survival.

With a few exceptions, all fish - including extinct ones - have backbones, a head (no neck), tail, fins, two eyes on the sides of the head and scales. They breathe via gills - an exception being ancient and modern lunged fishes that can breathe air and flop around on land. None had legs, arms, or even flippers.

Shubin reasoned in 1994, if mammals' earliest ancestors - amphibians - evolved from fish, where did limbs come from? He and two other paleontologists spent three brief arctic summers in Canada searching for ancient fossils to resolve that.

The first pushup

They found 365-million-year-old fossils of Tiktaalik roseae - a fish that had shoulders, forearms, elbows, wrists and fingerlike bones - that articulated (moved) like ours do. It could, like us, place palms (fins) flat on a stream bottom, flex the elbows and push up out of the water using large chest muscles to operate a rotating shoulder.

These modest 2- to 4-foot-long critters with eyes on the top of their heads were the missing link to amphibians, which possessed legs, necks and lungs, and were able to walk, breathe and hunt on land. Titkaaliks were the earliest creature on earth that could, as Shubin said, "Drop and give me 20 [push ups]."

He and his co-researchers reported in January that recently-discovered Tiktaalik fossils showed a pelvis and a joint for a femur to support walking back fins. Titkaaliks were rudimentary four-legged critters, tetrapods.

Why did Titkaaliks develop limbs?

Large fish in the shallow streams flowing through extensive mud flats during that time - geologically, the Devonian period - were 7- to 16-foot-long predators with railroad-spike-sized teeth. Surviving in waters of the Devonian period required that smaller fish either become bigger than their neighbors, develop armor plate against predators' teeth, or as Titkaaliks did over eons develop (evolve toward), limbs to take them where the big guys could not go - into shallows and onto mud flats. They started an evolution to land animals.